This invention relates to a novel process for the preparation of nano-crystallite forms of LTL, having crystal sizes less than 30 nano-meters. In particular, the zeolite has a structure of zeolite LTL and is prepared using stoichiometric quantities such that the ratio of product to reactants approaches unity, and the product crystals are less than about 30 nanometers diameter. The use of ammonia as a solvent or co-solvent in the process is essential to achievement of the desired product.
Ammonia is similar to water in that it is a dipolar, protonic solvent capable of acid-base chemistry. It has a dipole moment of 3.4.times.10.sup.-30 C.multidot.m compared to 4.9.times.10.sup.-30 C.multidot.m for the more polar water molecule. This results in ammonia being a poorer solvent than water for ionic substances, but a better solvent for more covalent compounds. At a pH greater than about 9, the equilibrium NH.sub.4 +OH.sup.- .fwdarw.NH.sub.3 +H.sub.2 O totally favors gaseous ammonia dissolved in the base solution. Pure ammonia has a vapor pressure of over 60 atm at 100.degree. C. compared to 1 atm for pure water. In this invention ammonia either promotes the "flood" nucleation of LTL or poisons the surface of crystals at the nucleus stage of between 100 .ANG. and 200 .ANG..
A zeolite designated as zeolite L is known to exhibit good catalytic properties, particularly for hydrocarbon conversion, and advantageous sorbent properties as described, for example, by Barrer et al., Surface Science, 12, 341 (1968). The chemical composition of zeolite L is disclosed in U.S. Pat. No. 3,216,789 to be: EQU 0.9-1.3(M.sub.2 /n):Al.sub.2 O.sub.3 :5.2 to 6.9SiO.sub.2 :xH.sub.2 O
where M is an exchangeable cation of valence n and x is from 0 to 9. Zeolite L also has a characteristic x-ray diffraction pattern, and its structure has been determined by Barrer et al., Zeit. Krist., 128, 352 (1969). The structure code LTL has been assigned to this structure type (Atlas of Zeolite Structure Types, 3rd Edn., W. M. Meier and D. H. Olson, Intl. Zeolite Assoc./Butterworths Press (1992)). The x-ray diffraction pattern of zeolite L has the following more significant d(.ANG.) values:
______________________________________ 16.1 .+-. 0.3 7.52 .+-. 0.04 6.00 .+-. 0.04 4.57 .+-. 0.04 4.35 .+-. 0.04 3.91 .+-. 0.02 3.47 .+-. 0.02 3.28 .+-. 0.02 3.17 .+-. 0.01 3.07 .+-. 0.01 2.91 .+-. 0.01 2.65 .+-. 0.01 2.46 .+-. 0.01 2.42 .+-. 0.01 2.19 .+-. 0.01 ______________________________________
A typical preparation of Zeolite L as disclosed by Breck, Zeolite Molecular Sieves, New York: J. Wiley, 283 (1974) employs an excess of SiO.sub.2 and a greater excess of K.sub.2 O. Isostructural compositions include ECR-2 (U.S. Pat. No. 4,552,731) and BaG-L (Baerlocher & Barrer, Zeit. Krist., v. 36, p. 245, (1972)).
The preparation of zeolite L described in U.S. Pat. No. 3,216,789 involves crystallizing the zeolite from a reaction mixture having a mole ratio of silica to alumina which is significantly higher than the ratio in the formed zeolite. Specifically, the reaction mixture comprises mole ratios:
______________________________________ K.sub.2 O/(K.sub.2 O + Na.sub.2 O) 0.33-1 (K.sub.2 O + Na.sub.2 O)/SiO.sub.2 0.35-0.5 SiO.sub.2 /Al.sub.2 O.sub.3 10-28 H.sub.2 O/(K.sub.2 O + Na.sub.2 O) 15-41 ______________________________________
Zeolite L and zeolites having related structures belong to the "L" family of zeolites. This family is characterized by having a 12-ring hexagonal structure with pore dimensions of about 5.5 to 7.2 .ANG.. In addition to zeolite L there are also barium zeolites Ba-G and Ba-G,L described by Barrer et al in J. Chem. Soc., 2296 (1964), J. Chem. Soc., 1254 (1972) and J. Chem. Soc., 934 (1974); ZSM-10 (U.S. Pat. No. 3,692,470) may be a DABCO containing member of this group of zeolites. Similarly, zeolite UJ (U.S. Pat. No. 3,298,780) may also be of the zeolite L type. Numerous syntheses of zeolite L have been reviewed in U.S. Pat. No. 4,973,461.
Structures have been proposed for zeolite L (Barrer et al, Zeit. Krist., 128, 352 (1969)) and GL (Baerlocher et al, ibid). If all cation positions in L are filled by monovalent cations, L will have a minimum Si/Al ratio of 1.8 according to Baerlocher et al (Zeit. Krist., v. 136, p. 253 (1972)). ECR-2 has an Si/Al composition in this range (U.S. Pat. No. 4,552,731).
Subsequently, several patents have claimed specific morphology LTL products, such as discs (EP 0096479), large cylindrical crystals (U.S. Pat. No. 4,544,539), and microcrystals (U.S. Pat. No. 5,064,630), and the subject of morphology variation and control has been discussed generally by Fajula (NATO ASI Series, v. 221B, p. 53 (1990)).
It has been found that zeolite L may be used as a catalyst base in aromatization reactions. U.S. Pat. No. 4,104,320 discloses dehydrocyclization of aliphatic compounds in the presence of hydrogen using a catalyst comprising zeolite L and a group VIII metal, in which the zeolite L is of the formula: EQU M.sub.2/n (AlO.sub.2).sub.9 (SiO.sub.2).sub.27
(where M is a cation of valence n), but the silica to alumina ratio may vary from 5 to 7.
East German Patent 88,789 discloses dehydrocyclization using a catalyst formed from a zeolite precursor with a silica to alumina ratio of 5 or greater which is dealuminized to give a silica to alumina ratio of up to 70. Zeolite L is mentioned as a precursor.
European Patent Application Publication 40119 discloses a dehydrocyclization process operating at low pressure (1 to 7 bars) or low H.sub.2 /hydrocarbon ratio using a catalyst comprising platinum on a potassium zeolite L. Belg. Patent 888,365 describes dehydrocyclization using a catalyst comprising platinum, rhenium (incorporated in the form of its carbonyl) and sulphur to give an atomic ratio of sulphur to platinum of 0.05 to 0.6 on a zeolitic crystalline aluminosilicate base such as zeolite L. Belg. Patent 792,608 discloses the treatment of zeolite L for use as a catalyst in isomerization by exchange with ammonium and chromium ions.